Sex on acid can be amazing, and as MGS said it can even be provoked by psychedelics.
But I agree with the advice of everyone else, that in this situation you shouldn't try for it. If it happens, it happens, but you should try to keep your mind elsewhere. People freak out even when they don't feel like someone is trying to get on them. You've also got to look out for not only what she wants then, but what you think she'll feel or regret after the trip ends.
I don't know the girl, but there's always the chance that while she liked you, she wouldn't have had sex had she not been tripping hard, and she'll realize it afterwards. Even if she doesn't hold it against you and all, still might make things awkward. Connecting on a personal level is the best bet if you really like her, and it could pave the way for a great relationship. I don't mean to sound like I'm shaking my finger at the idea of sex here. But if you want to play it completely safe and simple, and still really beneficial, I'd keep the physical stuff pg-13ish to be safe until the comedown. Never know if you'll let some demons loose in the worst possible way.
My last relationship, of 3 years, started out in such a way. You can really feel like you've known someone for years after one night of bliss or even terror or any other emotions heightened by psychedelics.
